In what appears to be a masterful illusion, astronomers at Gemini
Observatory have imaged two nearly identical spiral galaxies in Virgo,
90 million light years distant, in the early stages of a gentle
gravitational embrace. The new image was obtained at the Gemini South telescope in Chile using GMOS, the Gemini Multi-Object Spectrograph.

Like two skaters grabbing hands while passing, NGC 5427 (the nearly
open-faced spiral galaxy at lower left) and its southern twin NGC 5426
(the more oblique galaxy at upper right), are in the throes of a slow
but disturbing interaction – one that could take a hundred million
years to complete.

At a glance, these twin galaxies -- which have similar masses,
structures, and shapes and are together known as Arp 271 – appear
undisturbed. But recent studies have shown that the mutual pull of
gravity has already begun to alter and distort their visible features.

Typically, the first sign of a galaxy interaction is the formation of a
bridge-like feature. Indeed, the two spiral arms on the western (upper)
side of NGC 5426 appear as long appendages that connect with NGC 5427.
This intergalactic bridge acts like a feeding tube, allowing the twins
to share gas and dust with one other across the 60,000 light years (less
than one galaxy diameter) of space separating them.

Colliding gases caused by the interaction may have also triggered bursts
of star formation (starbursts) in each galaxy. Star-forming, or HII,
regions appear as hot pink knots that trace out the spiral patterns in
each galaxy. HII regions are common to many spiral systems, but the
giant ones in NGC 5426 are curiously knotted and more abundant on the
side of the galaxy closest to NGC 5427. Starburst activity can also be
seen in the galaxy’s connecting bridge.

Likewise, the giant HII regions in NGC 5427’s disk are forming at
a higher rate, and are more plentiful, than expected for a galaxy of
this type. One giant star-forming region at the tip of NGC 5427’s
western (top) spiral arm, looks especially large and disturbed, as does
the arm itself, which is unusually straight, as if strong tidal forces
have broken the arm in two, causing it to bleed starlight.

Despite their appearance in this two-dimensional image, NGC 5426’s
western (top) spiral arm is the one closest to us, as opposed to NGC
5427’s southeastern (bottom) arm. NGC 5426 is also the closer of
the two galaxies. Over millions of years, however, NGC 5427 will perform
a parabolic traverse (see Figure 2), moving it from behind NGC 5426 towards the
foreground in the upper-right corner of the frame. Thus, an imaginary
long-lived observer on a planet in NGC 5427 would see an almost
perpendicular passage of the companion galaxy.

Once thought to be unusual and rare, gravitational interactions between
galaxies are now known to be quite common (especially in densely
populated galaxy clusters) and are considered to play an important role
in galaxy evolution. Most galaxies have probably had at least one major,
if not many minor, interactions with other galaxies since the advent of
the Big Bang some 13 billion years ago. Our own Milky Way, a spiral
galaxy like those in this image, is, in fact, performing its own stately
dance. Both with the nearby dwarf galaxy, called the Large Magellanic
Cloud and a future interaction with the large spiral galaxy M-31 or the
Great Andromeda Galaxy, which is now located about 2.6 million light
years away from the Milky Way. This new Gemini image is possibly a
preview of things to come for our own galaxy. Ultimately the end result of these types of collisions will be a large elliptical galaxy.

The Gemini Observatory is an international collaboration with two identical 8-meter telescopes. The Frederick C. Gillett Gemini Telescope is located on Mauna Kea, Hawai'i (Gemini North) and the other telescope on Cerro Pachón in central Chile (Gemini South); together the twin telescopes provide full coverage over both hemispheres of the sky. The telescopes incorporate technologies that allow large, relatively thin mirrors, under active control, to collect and focus both visible and infrared radiation from space.

The Gemini Observatory provides the astronomical communities in five partner countries with state-of-the-art astronomical facilities that allocate observing time in proportion to each country's contribution. In addition to financial support, each country also contributes significant scientific and technical resources. The national research agencies that form the Gemini partnership include: the US National Science Foundation (NSF), the Canadian National Research Council (NRC), the Argentinean Ministerio de Ciencia, Tecnología e Innovación Productiva, the Brazilian Ministério da Ciência, Tecnologia e Inovação and the Chilean Comisión Nacional de Investigación Cientifica y Tecnológica (CONICYT). The observatory is managed by the Association of Universities for Research in Astronomy, Inc. (AURA) under a cooperative agreement with the NSF. The NSF also serves as the executive agency for the international partnership.